Human motion requires exertion of energy. Peoples' ability to conduct their activities can be limited by their available energy. For example, hikers have a limit to the distance they can hike based upon their physiological constitution and condition. Runners have a limit to the speed they can run. Military troops have a limit to the distance they can march, for example, with a heavy pack load. Athletes have a limit of how long they can remain within a physiological envelope of control that allows them to maintain adequate resilience to injury. People often seek ways to extend their capabilities—to run faster, hike farther, jump higher, stay more resilient, etc. It would be desirable to extend people's capabilities and overcome some of their limitations.
It is known generally that a device can receive a force and store potential energy. Later, the device may be actuated to release the potential energy as kinetic energy. During dorsiflexion motion of the ankle and lower leg system of a user, force acts over a distance and potential energy is stored in a force/energy management system according to the several footwear embodiments described herein. The stored potential energy is then returned to the ankle and lower leg system as kinetic energy during plantar flexion motion. With the assistance of such force and energy, a person is less dependent upon internal muscles, flexor tendons and tendons for locomotion and stability. The person can perform better, experience less fatigue and be able to maintain an envelope of control which provides sustained resilience to injury, recuperate from lower limb issues faster and receive other health and performance benefits.
Gait Cycle
Human locomotion is driven by three major energy sources—the foot system, the knee system and the hip system. Each of these systems is moved by a combination of muscle force as well as tendon force. In a typical walking gait, roughly 40 to 45% of energy is provided by the foot system, which surpasses the individual contributions of both the knee and hip systems. As stride length or gait speed increases the relative contribution of the foot system decreases in relation to the knee and hip system.
During a gait cycle, as the term is used herein, the Achilles tendon stretches during dorsiflexion motion and releases during plantar flexion motion. The efficiency of the Achilles tendon is quite high, with laboratory measures showing a potential for a greater than 90% energy return. The Achilles is an elastomeric element that is capable of stretching up to 8% of total length under load before plastic deformation.
The use of powered exo-skeletons has been demonstrated in the laboratory; (reference may be made to articles cited in the attached bibliography, incorporated by reference herein as to any material deemed essential to an understanding of the principles of energy management disclosed herein). The use of powered exoskeletons for the ankles has been tested on the treadmill and showed to have potential to enable improved performance. These studies also show that managing the timing of the release of energy from these powered systems requires some learning on the part of the wearer. Proper harmonization of the device with the gait cycle is a necessity for a person to gain significant benefit.
Because of these tests, supplementing the foot system with support and added energy capability through an external system can be meaningful. A supplemental system can help athletes perform better. Such a system can help boost walking endurance; it can help people with ankle and Achilles tendon injuries recuperate faster and help avoid future problems. Also, it can help people walk more easily and with less fatigue. Such a system should also be timed correctly to harmonize with the proper need for energy.
Plane of Reference
Performance benefits that may be achievable using a supplemental system include improved speed, improved endurance, increased jump height, increased backpack loading, decrease in oxygen consumption, etc. A focus of such a system may be on the rotation of the ankle joint in the sagittal plane as a main source of force and energy.
Benefits may also be achieved by such a supplemental system in the frontal plane. In shoe structural design, the frontal plane may be utilized to maintain or extend a shoe's protective capabilities in the ankle and limit range of untoward varus or valgus motion in the ankle that may otherwise lead to sprain or other injuries.
Typical Biomechanics of the Human Ankle
A typical human ankle range of motion is commonly discussed in biomechanics literature with variations according to each authors' clinical experience; the following overview of the normal gait cycle is a simplified recounting of common literature.
The gait cycle may begin with the first touch of the foot to the ground. This first touch begins the cycle at 0% and the moment immediately prior to the following touch to the ground of the same limb may represent 100% of a cycle. In the normal walking gait, the ankle may experience a small amount of extension after initial contact leading to plantar flexion during the first 10-15 or so percent of the cycle, commonly referred to as a loading response. This is then followed by increasing amounts of dorsiflexion motion, which further increases after mid-stance. Maximum dorsiflexion is typically achieved after heel lift and prior to the initial contact of the opposite foot. This is followed by rapid plantar flexion motion associated with push off, which occurs after the opposite foot makes its initial contact. In the push-off phase, the ankle plantar flexes through toe off. This is followed by a swing phase with the foot traveling in the air. During the swing phase, the foot dorsiflexes to a neutral position preparing it for the next cycle.
For simplicity in writing of the patent, we will refer to ankle system motion during the periods of increasing flexion after initial contract and loading response, through mid-stance, through heel lift, to peak dorsiflexion as “dorsiflexion”; and we will refer to ankle system motion during the periods of increasing extension found during opposite foot contact through toe off as “plantar flexion”.
The total range of motion in the ankle during a walking gait is the result of a combination of dorsiflexion angle and plantar flexion angle. After midstance, there is increasing dorsiflexion to a peak of 5 to 15 degrees as measured according to well known technical arts. During push off, the ankle rapidly plantar flexes to a peak of −5 to −20 degrees. Typical total range of motion during the normal walking gait is often shown as 20 to 40 degrees in common literature and internet resources.
Analyzing the running gait where a walking gait has been discussed above, we see similar elements of the cycle; however, efficient runners rarely land on their heels in order to prevent unnecessary losses in energy. Rather, initial contact is on the front part of the foot while the ankle is in slight dorsiflexion. The amount of dorsiflexion increases after midstance to a peak of 20 to 50 degrees. This is followed by rapid and powerful push off during which the ankle plantar flexes to a peak of −10 to 30 degrees. This results in a total range of motion of 40 to 70 degrees. Jogging gaits may range between the walk and run depending upon the person jogging, their abilities, the conditions, their level of exertion, etc. Sprinting gaits often show a decrease in range of motion when the athlete is near the top of their speed range.
Benefits of External Assistance During Dorsiflexion
When an ankle is in dorsiflexion phase, with a joint angle greater than zero, some amount of force needs to be applied to keep the ankle joint angle from rapidly increasing which would lead to the joint collapsing under the weight of the body. The removal or full rupture of the Achilles tendon and removal of other supportive ankle muscles & tendons, for example, would result in joint instability and the inability for a person to bear their body weight upon that foot. Any amount of dorsiflexion results in a necessary force being exerted in the ankle region to prevent joint collapse. A reduction in the force necessary to support the body during dorsiflexion phase, therefore, can be perceived as a potential opportunity to save energy or boost performance.
Several inventors have attempted to use differential forces above and below the ankle joint in the past to produce inventions that would be helpful to people. For example, Borden, U.S. Pat. No. 5,090,138, discloses a spring shoe device with a heel socket, shin brace, ankle hinge and spring strap. Stewart, U.S. Pat. No. 5,125,171, discloses a shoe with a spring biased upper. Frost, U.S. Pat. No. 5,621,985, discloses a jumping assist system with multiple components. A rather elaborate design is disclosed by Seymour, U.S. Pat. No. 6,397,496, for an article of footwear which employed multiple springs to assist motion of a boot in the upward direction.
A distinct limitation of the current art is that the elements do not appear to be successfully integrated into the upper or collar of a shoe such that human locomotion is improved, for example, with both an improvement in a rotation zone and an elastic zone. Furthermore, cuffs designed for going over the lower leg to the extent present in the art are not integrated into the aesthetics of common footwear.
The known technical art fails to simplify structural elements of a device above the ankle to receive force and transmit the force to a spring. Exemplary art may show a device which depends upon non-trivial collars that wrap the leg above the ankle, the bulk of which contributes to their inability to be effectively integrated into traditional footwear. Similarly, anchors below the ankle, to the extent depicted in the known technical art, are often shown as appendages and extraneous devices which may interfere with preferred shoe design techniques.
In view of the prior art, there is a need to minimize the complexity, cost, weight and materials used to enable an article of footwear to harvest energy from the lower leg.
Summary of the Structural Elements of the Several Footwear Embodiments
The embodiments of footwear described herein improve upon the known art of footwear design in many respects, including clever management of forces from the lower leg into a shoe using familiar shoe design approaches, tooling, materials and manufacturing approaches. An intention of the several embodiments and structural elements thereof disclosed herein (sandals excluded) is to create footwear with performance improvements integrated into the design, aesthetics, material selection and construction so that they can be successfully commercialized. Examples of prior art have relied upon appendages, additions and changes to footwear construction and material selection that have not reached commercial viability.
The several embodiments (sandals excluded) integrate their novel improvements in a way that enables the footwear to avoid being perceived as a contraption, and provides aesthetic shoe designers with a design palate that enables them to offer a wide range of ornamentally inspiring designs.
Force above the ankle is exerted predominantly by the pressure of the front surface of the lower leg upon a receiving device such as a tongue of a high top collar of a shoe or boot. To achieve an upward stretch of a tension spring in proximity to the Achilles, one must use some type of mechanism to change direction of the force from near-horizontal to near-vertical. Prior art examples typically relied upon cuffing of the lower leg, which can lead to discomfort, unnecessary size, unnecessary weight, and unnecessary banding forces around the perimeter which may unduly constrict motion of tendons, ligaments, blood flow, and the Achilles tendon itself. Collar mechanisms put unnecessary force upon the rear of the leg, which has no capability of delivering primary forces. The embodiments herein and aspects thereof demonstrate a variety of ways in which forces may be managed without undue cuffing forces, especially to the rear of the lower leg.
Bilateral Components in Depicted Footwear
It is assumed in the descriptions of embodiments and by the depictions thereof in the drawings showing but one side view herein that the user of skill in the art will be aware that many of the components mentioned are bilateral in nature, with both medial and lateral instances. As an example, there are typically two eyestays in each shoe, a medial eyestay and lateral eyestay. By assuming this knowledge, plural terms are not used herein and so eliminate the need for specifying medial and lateral instances of bilateral components.
To be clear, it is known in the art that bilateral components may not be mirror images or exact copies of each other. For example, the ankle joint is not horizontal to the ground, and the medial side is higher than the lateral side. Those skilled in the art will be able to still gain clear understanding of these teachings by limiting descriptive language to the singular.
Using Stretch of a Passive Energy Storage Device to Manage Energy
In powered external foot/ankle exoskeletons, motive force may be provided by pneumatic cylinders. In shoe embodiments described herein, a passive energy storage device is used to manage forces and energy external to the body. A passive device structural element of the several embodiments of a shoe as described herein may include a spring, elastic member, elastomeric component or other such device known in the art, particularly located according to the figures.
Thus, the several embodiments involve the storage and management of energy under tension. Tensile energy may be stored and released in any variety of commonly used formats, such as an elastic cord or multiple cords, coil spring, an elastic band, a bungee cord, a an elastomeric material, a woven cord, etc. Energy may also be stored in a planar or sheet surface. Sheet materials such as latex sheets, flat latex bands, rubber sheets, rubber tubes, woven fabrics, non-woven fabrics, etc can all apply force, store energy and release energy when tension is applied to them. Tensile energy may also be stored and released in custom-shaped or molded elastomeric objects such as a set of cords overmolded into a common element, or molded elastic elements that contour to the outside of a shoe or the rear of a foot, ankle and leg. Molding of rubber, thermoplastic rubber or urethane, silicones, and other elastomerics are common in footwear and can be applied herein.
A wide variety of shapes, a small number of examples which are described above, will henceforth be noted as tension springs. Reference to tension springs therefore will broadly address a variety of materials and shapes that can act in tension.
Benefits of Tension Spring Force/Energy Management During Dorsiflexion and Plantar Flexion
During the walking gait cycle, the peak demand for ankle energy occurs after midstance as the ankle is in the process of increasing dorsiflexion and then rapidly plantar flexing. The transition of decelerating dorsiflexion motion to accelerating plantar flexion motion requires the contribution of the Achilles tendon and the soleus and gastrocnemius muscles as well as a variety of other muscles and connective tissues including tendons. The Achilles tendon can stretch up to 8% before plastic deformation.
While the Achilles tendon is a very efficient member, capable of returning more than 90% of energy stored within, associated muscle is not as efficient. Use of the muscle in the gait cycle is consumptive of energy. Literature shows that during the period of dorsiflexion, the ankle system consumes approximately 0.2 to 0.5 W/kg of power, while during the time of transition from dorsiflexion to plantar flexion the ankle system consumes roughly 2 to 4 W/kg of power.
By anchoring a tension spring to capture range of vertical motion or diagonal motion, as described below, one can impose a force during dorsiflexion which harvests energy for each degree of ankle rotation in the dorsiflexion direction. This externalizes force outside of the body and stores energy as potential energy.
By externalizing force and energy during dorsiflexion, several things are accomplished: reduce the amount of muscle force and energy required to manage dorsiflexion (and prevent the collapse of the joint) thereby reducing the power requirement, typically shown as 0.2 to 0.5 W/kg; reduce the total energy needed to be managed and stored by the tendons; and either reduce oxygen consumption assuming a steady gait or provide an opportunity for a more aggressive gait without additional oxygen demand. Similarly, the energy stored in the tension spring may be returned to assist in plantar flexion motion by applying force across a distance.
By converting the externalized potential energy into force that is internalized into the foot, several things are accomplished: reduce the amount of muscle force and energy required to manage plantar flexion (and provide forward gait propulsion) thereby reducing the power requirement, typically shown as 2 to 4 W/kg; reduce the total energy needed to be managed and stored by the tendons; either reduce oxygen consumption assuming a steady gait or provide an opportunity for a more aggressive gait without additional oxygen demand; and assist in a variety of other ankle mediated tasks, such as jumping, hopping, leaping, etc.
Simplified View of a Shoe System Involving Structural Elements of the Several Shoe Embodiments
The structural elements of the several show embodiments disclosed herein exploit differentials between the foot system below the ankle and the leg system above the ankle. In order to perform mechanical work, a force is applied over a distance. Therefore, in order for the systems to work, we identify means for anchoring force-carrying devices so that force can be applied, and we identify means to harvest this force over a range of motion distance.
Simplified View Regarding Leg Force Below the Ankle
Forces are managed in the several depicted embodiments by establishing anchors integrally within footwear, for example, below the ankle and above the ankle of the wearer of depicted footwear.
Anchoring forces below the ankle is accomplished with the aid of an article of footwear. Because the foot is wrapped on many surfaces by an article of footwear, force can be transferred effectively and distributed broadly to ensure comfort.
Force carrying members, anchors and supplemental means of support into footwear of the several embodiments such that a shoe manufacturer or maker may maintain geometrical stability in the footwear and anchor, comfort to the user, adequate aesthetic appeal to the buyer, cost that is appropriate for the application, longevity commensurate with the application, lightness of weight, safety, among various other concerns necessary for a commercially viable product.
Simplified View Regarding Leg Force Below the Ankle
Anchoring forces in and out of the lower leg above the ankle is one aspect of the several show embodiments. Another is to apply the fore and aft force to the front face of the lower leg which may create a force to assist plantar flexion motion of the foot and conserve energy during dorsiflexion motion of the foot.
In addition to the fore and aft force applied to the lower leg, there are also other forces that act upon a lower leg device. In the several embodiments, a rotational force may be directed into lifting the heel of the user and driving plantar flexion. As such, there is an equal and opposite downward force on the lower leg which is managed. As this is a dynamic system which is also influenced by the accelerations based upon the knee and hip systems as well as environmental factors and the influence of human activity, various other forces will exhibit themselves throughout any given activity.
To integrate an adequate lower leg anchoring system within an article of footwear, the several embodiments and aspects thereof disclosed herein will use two approaches both independently and in combination within articles of footwear. Several terms need to be defined for clarification of the several embodiments.
Yoke—a yoke is defined for this application as a device which relies upon managing forces on three active sides through a “U” shaped configuration. Herein, the base of the “U” is positioned against the front face of the lower leg and is able to receive fore and aft forces. The lateral and medial sides of the “U” are positioned near horizontally above the malleolus ankle bulge and able to manage up and down forces through skin friction as well as interference with bony malleolus ankle bulge, as well as through integration with a pivot system in proximity to a rotation axis of the ankle. There may be a 4th side of a yoke device that connects the open legs of the “U”, however, this side is often not responsible for carrying primary forces.
Collar—a collar is a band that constricts the outer diameter of an object it encircles. It can apply a vertical force on the leg through a combination of skin friction resistance as well as a mechanical force when the inner diameter of the collar is smaller than the outer diameter of the bony protuberances of the ankle it encircles.
Collar yoke—a combination of the U-shaped yoke together with a circumferential band or collar, the design of which can distribute primary forces, secondary forces and disparate other forces to specific areas of the device, as well as manage rotational and pivot forces.
Simplified View Regarding Range of Motion
To manage force and energy, the novel concepts herein integrate elements into footwear to establish anchor points and mechanisms which spread a tension spring further apart from plantar flexion to dorsiflexion as well as manage rotational and pivot forces.
There are two areas of expansion that the several embodiments may exploit (independently and in combination): 1) a range of motion vertically, roughly parallel to the Achilles, which is managed through employing a rotatable collar yoke that has a hinge point in proximity of the ankle joint and translates near-horizontal pressure force from lower front of the leg over a fulcrum and into a near-vertical force on a tension spring at the lower rear of the leg; and 2) a range of motion diagonally from shin to heel, which is carried by a collar lobe, yoke or collar yoke that can rotate and or move linearly forward and backward thereby transferring near-horizontal pressure force from the lower front of the leg to a near-diagonal force on a tension spring which is attached on its opposite side to an area that is above the top rear of a heel counter of a shoe.
Simplified View of Exploiting Range of Motion Vertically
To measure vertical expansion and contraction, one can place ink marks on the lower limb along the Achilles tendon. During the range of motion found in dorsiflexion and plantar flexion in a gait cycle, the distance between these reference points will vary by several centimeters. This change in distance is mediated by the combination of changes in length of several bodily members, including the Achilles tendon, the calf muscles including the soleus and gastrocnemius muscles.
This change in length of these major members is distributed over their combined working length, which in an adult can be over 35 cm in total length. External to the body, however, this change in distance between our two illustrative ink marker points on skin is not evenly distributed across this combined length. Inspection of the skin in the region of the Achilles tendon shows that the majority of stretching and compression of the skin surface is associated with a small region.
The region of the posterior face of skin over the Achilles tendon that is posterior to the ankle shows a high degree of skin stretch and compression. This region can be approximated in an adult as starting at 5 cm in height above the floor at an upright standing position and continuing up to 10 cm in height above the floor. The skin in this region is often wrinkled, showing the history of significant stretching and compression over years of use. We will henceforth refer to this area as the “creased skin region”.
The creased skin region can be roughly described as a triangular or wedge shape. The axis of ankle rotation defines the anterior point of the wedge. Two imaginary lines emanate from the axis of ankle rotation to the anterior upper and lower limits of significant skin stretch and compression. By way of example, the upper line may be roughly 5 cm in length and the lower line may be 6 cm in length. The imaginary near vertical 5 cm line between these two points define the hypotenuse of the triangle. Skin will stretch and compress outside of this region, but the majority of skin stretch and compression is observed in this region.
To illustrate the potential for range of motion across the creased skin region, one can imagine that this region may be measured at 5 cm in length as measured along a vertical axis when standing upright and still. During dorsiflexion, this length may stretch to 7 cm or more in length. During plantar flexion, this length may compress to 3 cm in length or less. This results in a range of linear expansion/contraction total of 4 cm or more.
Enabling Vertical Range of Motion
Unfortunately, there is no convenient physical bodily feature upon which to directly anchor a force carrying object to the rear face of the lower leg above the creased skin region. A feature of the embodiments herein is to enable such functionality in footwear.
One approach is to cuff the lower leg, such that the cuff stays stable on the lower leg and provides a means for anchoring a mechanical attachment at the back of the cuff.
Various collar mechanisms were experimentally fitted around the lower leg to determine the ability for using cuffs that impinged upon the protrusions of the ankle (lateral & medial malleolus) as a way to keep the cuff stable and manage downward force. Examples of this type of cuff are seen in gymnastics grips which use the bulge of the wrist bones as a means for anchoring hand grips. Gymnastic grips can manage over a thousand Newton, leading to a hypothesis that a similar collar around the lower leg could manage similar forces.
It has been experimentally determined that a tight collar around the ankle could easily support a large amount of force, but that the application would also be influenced by the duration of use and the amount of discomfort accepted by the user. The higher the force, the higher the discomfort. Cuffs that are unusually large may distribute forces more broadly, but may not enable required footwear performance or be aesthetically acceptable. There is also an issue of interference with the rear tendons of the lower leg. The nature of a collar is to constrict an object within its diameter. If an object that is being encircled by a collar has a protuberance, it will receive a greater amount of the collaring force. As such, collars placed immediately above the malleolus tend to place a significant amount of force on the Achilles region, leading to discomfort, abrasion and pressure points. This is worsened by the ongoing cycle of stretching and relaxation of the Achilles which can allow the collar to seat itself each time the tendon is relaxed and then constrict when the tendon is in tension.
Gymnastic routines upon rings or bars last only a matter of one or two minutes, enabling the athlete to tolerate discomfort in exchange for the benefit offered from improved performance. Similarly, specialty footwear applications in which users can accept discomfort for a brief time may allow the disclosed embodiments to apply significant collaring forces above the malleolus. However, for the majority of applications, users will desire a solution which is comfortable over the duration of the time the footwear is worn using a sufficiently small collar arrangement to properly integrate with their footwear. As such, the amount of downward force that can practically be managed by collaring above the malleolus should be limited.
Since there is a practical limit of the amount of force that can be managed through collaring forces above the malleolus ankle bulge, there is an unmet need to supplement or replace collar based force management. Other mechanisms have been considered in the past that employ garters around the upper calf, knee area and even the hip area. As these have never been successfully commercialized, these are considered impractical. Other mechanisms have been considered which employ a very large cuff around the ankle as common with orthopedic braces. These too have never been adopted into the footwear market and are considered impractical.
An approach to exploit vertical range of motion taught herein is to integrate into footwear an articulating member which enables forward motion of the lower leg into a yoke-based device that is then transferred over a fulcrum to enable a vertical force and motion upon a spring.
A yoke or collar yoke arrangement is described in several embodiments which enables management of primary forward leg force from contact with the lower leg, pivot force from contact with a fulcrum point in proximity to the ankle joint, and downward force from contact with a spring element. Additionally, features are discussed which enable the system to have sufficient stability against secondary forces to maintain viability within the application and within aesthetic and other design limitations.
In particular, an open yoke sandal embodiment demonstrates that force carrying efficacy within footwear can be accomplished without unnecessary cuffing or collar forces. This enables function of the system without unnecessary pressure on the skin in the Achilles region. The integration of a yoke into a collar to produce a collar yoke is another novel concept. In this manner, primary forces from the lower leg can be managed through the yoke functionality within a collar. This enables management of significant primary force and ensuing torsional forces over the pivot without at a high degree of banding force of the collar. As such, significant force can be managed at the front of the lower leg without unnecessary pressure upon the Achilles tendon area at the rear surface of the lower leg. The benefits of a banded high collar for aesthetics, management of untoward varus and valgus motion in the ankle, management of environmental forces and other protective benefits may be maintained. The length of the side walls of the yoke members may also be slightly elongated to the rear, thereby creating an eccentric (i.e.: oval) shape to the collar, which can reduce the banding upon the rear of the lower leg.
Simplified View of Exploiting Range of Motion Diagonally
As described below, a region superior to the ankle joint that extends diagonally from the front face of the lower left to the top of the heel can experience a change in diagonal length of 2.5 cm or more during a gait cycle. By applying an external tension spring in this region, we can store and return significant energy.
To measure diagonal expansion and contraction, one can place ink marks on the lower limb along the base of the shin as well as the bottom of the creased skin region along the Achilles tendon. During the range of motion found in dorsiflexion and plantar flexion in a gait cycle, the distance between these reference points will vary by several centimeters.
This change in distance is relative to the elevation of the front anchor point. If the superior anchor point is placed at the base of the shin all the way down to an elevation level with the horizontal plane of the ankle joint, there is only minimal change in distance between it and the inferior anchor near the heel.
As the superior anchor point is elevated along the base of the shin, the change in distance between dorsiflexion and plantar flexion can reach over 2 cm. Common high top basketball shoes reach up 16 to 18 cm off the floor. Assuming that the horizontal plane of the midpoint of the ankle joint (which is not level to the ground) is roughly 11 cm off the ground, one can visualize that the top of the front of a common high top collar or tongue reaches 5 to 7 cm above the ankle joint elevation.
Thus, by establishing a superior anchor point near the top of the front of a high top collar and the inferior anchor point above the heel counter of a shoe, that there is an opportunity to observe a 2 cm or more change in distance across dorsiflexion and plantar flexion.
Spring Design and Geometry
As mentioned above, springs of a variety of materials and shapes may be utilized in the several embodiments. Springs may also be designed in parallel with other materials, such as straps or stiffer springs, which can limit range of motion. In doing so the spring may stretch out to a certain extent and then be limited by the other material. This may help prevent untoward motion.
The geometry of the device within a shoe will also determine the starting point at which the force may be exerted. This geometry will establish the range of motion in which the spring is not yet active and the range of motion in which the spring or springs are active. For example a geometry can be constructed to be helpful to people who do not wish their shoes to induce plantar flexion angle beyond neutral—for example people with limited ankle strength. Spring force would increase linearly in dorsiflexion from 0 to 30°, but there would be no spring force in plantar flexion at less than 0°. For example, a walking shoe may benefit from having spring force linearly increase starting at −5° and ranging to 25 or 30°.
Or, for example, a person engaging in an athletic sport may wish to have spring force start at minus 20° and increased linearly through positive 40°. This would tend to position the foot in a plantar flexion position during the swing phase and help the athlete maximize the amount of energy storage at each step. The spring force could also be designed non-linearly so that there is a light spring force from minus 20° to 0°, and then an increased spring force from 0 to 40°.
Varying Spring Force with Shoe Size
The several embodiments disclosed herein may be of benefit to people of all shoe sizes. While there is no direct correlation between shoe size and body weight of any given individual, one can make a generalization across the population that body weight increases with shoe size. Therefore, the larger the shoe, the higher the spring rate designed into the system.
Increase in body weight will benefit from an increase in spring rate. A linear progression will enable this adjustment, for example Spring Rate=Design Factor×Shoe Length. For example, a Design factor of 1.2 N/cm2 for a 16 cm Foot Length will yield a 19.2 N/cm Spring Rate for a shoe size that is roughly 8.5 in US sizing; while the same Design Factor of 1.2 N/cm2 for a 20 cm Foot Length will yield a 24 N/cm Spring Rate for a shoe size that is roughly 13 in US sizing. Design factors will be different for adult ranges of sizes versus youth ranges of sizes.
Comfort is limited by undue pressure. Correlating spring rate linearly to foot size can help ensure that pressure is also managed properly. Pressure upon the front face of the lower leg is calculated as a function of the surface area of the yoke face upon the lower leg, which nominally equals lower leg width times yoke breadth. Assuming that lower leg width is nominally associated as a linear function of foot size across a population, and that the breadth of the yoke will increase linearly with foot size, then the available surface area will increase geometrically with foot size. This increase in yoke surface area will accommodate a linear correlation of spring rate to foot size, assuming that the Design Factor is maintained nominally between 1 and 2.
Timing
Studies using powered ankle exoskeletons showed that the timing by which power was delivered from the exoskeleton into the ankle system was a significant variable in determining the performance of the wearer. Improper timing led to poor performance and proper timing required conscious effort by the user.
Similarly, in many heel-based energy management systems, energy can be absorbed upon initial contact of the heel to the ground, but the timing of the return of energy can impact resulting performance. The return of energy out of a heel based spring/cushion system is often delivered too quickly to be of significant performance benefit to the user.
A feature of the embodiments disclosed herein is in their ability to harmonize force/energy capture and energy return with the wearer's gait cycle. Proof-of-principle experiments with rough prototypes show an improvement in performance which exceed initial estimates. One hypothesis for this unanticipated benefit is that the force/energy management systems disclosed herein have functionality which is similar in behavior to internal tendons, and so can complement their activity synchronously throughout all of dorsiflexion and plantar flexion as well as rotation.